This invention relates to a method of applying a fluoropolymer film to a body and to bodies so treated.
Oleophobic or superhydrophobic surfaces are desired for a number of applications. The invention arises out of investigations of the phenomenon of surfaces with lower energy than ptfe (polytetrafluoroethylene) by taking advantage of the effect arising from attachment CF3 groups to a variety of materials.
The invention may be applicable to thin films usable in polymeric filter media and to cold plasma treatments to create low energy surfaces upon low-cost thermoplastics and natural media, and to the functionalisation of fluorinated polymers such as PTFE and PVDF (polyvinylidene difluoride). This specification discusses a plasma procedure leading to a thin film of perfluoroalkyl groups upon a substrate, which will exhibit superhydrophobicity or oleophobicity. By this we mean that the surface will repel liquid with surface energies as low as that of acetone and alcohol.
The controlled deposition of many plasma polymers has been examined and the ratio of CF2 to CF3 is documented in terms of monomer type, plasma power levels and proximity to the glow region. We are now describing a new method for creating surfaces with greater coverage of functional groups which offers a novel approach to the creation of polymer surfaces by pulsed gas introduction of the plasma.
According to the present invention, a method of applying a fluoropolymer film to a porous or microporous or other body, comprises exposing the body to cold plasma polymerisation using a pulsed gas regime to form either (i) an adherent layer of unsaturated carboxylic (e.g. acrylic) acid polymer on the surface and then derivatising the polymer to attach a perfluoroalkyl group terminating in xe2x80x94CF3 trifluoromethyl. A combination of electrical and gas pulsing may be used.
Preferably, the cold method of applying a fluoropolymer film according to 1 and 2 wherein the cold plasma polymerisation uses an unsaturated carboxylic acid.
The xe2x80x9cgas onxe2x80x9d and xe2x80x9cgas offxe2x80x9d times are preferably from 0.1 microsecond to 10 seconds.
The pulsed gas may be oxygen, or may be a noble or inert gas or H2, N2 or CO2. Alternatively, acrylic acid polymer precursor may be pulsed directly without a process gas.
The body may be a film (not necessarily microporous) or of other geometry that allows coating by plasma polymerisation to a standard of consistency adequate for the end use.
The method may be stopped at any stage, when the applied film is continuous and impervious or at an earlier stage, when it is to a greater or lesser extent still apertured, i.e. has not yet completely filled in the underlying pores of the body. The pore size of the finished product can be set to any desired value by ceasing the method after an appropriate duration.
The plasma power is preferably 1W to 100W, more preferably 1.5W to 7W.
The invention extends to the body with the thus-applied film. The substrate material of the body may be carbonaceous (e.g. a natural material such as cellulose, collagen or alginate, e.g. linen), synthetic, ceramic or metallic or a combination of these.
Electrical pulsing of the radio frequency supply to the plasma is known. This technique can endure a more rapid deposition and greater coverage of the substrate surface by the plasma polymer. We have utilised the plasma polymerisation of acrylic acid, which again is known but using a pulsed gas regime and clearly there are many other possible unsaturated carboxylic acids available as monomers. It is believed that such functionalities impart a degree of biocompatibility to substrates and allow of call culture experiments to be undertaken successfully upon such a surface even with difficult an sensitive cell lines.
By virtue of a derivatisation stage, the acid group may be reacted with a range of materials, for example perfluoralkylamines, to yield a surface rich in perfluoroalkylamide groups. In this way the surface would predominate in CF3 functions. Additionally the use of fluorinated surfactants will similarly generate a surface film of lower energy than ptfe and find application in for example the packaging market where oleophobic materials are desirable.
In the packaging market, there is a need for oleophobic venting films where the contents of a vessel or a package may require the release of differential pressure. Such pressure differentials may arise from expansion or contraction of the container or the liquid contents, with changes in the ambient temperature or pressure. The liquid contents must be retained without leakage and so porous venting aids are used. In those situations where liquids of low surface tension are involved e.g. surfactants, detergents, or organic solvents, then conventional porous ptfe materials are not as efficient. The surface energy of such materials is of the order of 18 to 20 dynes/cm at 20xc2x0 C. and the energy of a CF3 surface is less at perhaps 6 dynes/cm, and can be influenced by the plasma conditions used for the deposition. It is also known that the substrate morphology can influence the value of the contact angle since surfaces of a certain roughness can lead to composite angels. The surface which has the greatest number of CF3 groups packed together will have the lowest surface energy.
Products having superior (high density) surface coverage, rapidly deposited, may arise from gas pulsing alone or in combination with R.F. pulsing. Such materials have application in filtration, chromatography, medical device and laboratory ware. For example low cost thermoplastics could be coated using perfluorocarbon monomers to afford ptfe-like properties.
The body or substrate upon which the superhydrophobic layer is attached may be a carbonaceous polymer, e.g. a fluoropolymer such as ptfe, optionally itself a film, which may be porous or microporous. The process can also be applied to other polymers such as polyethylene and a range of other materials used for the biocompatible properties conferred by the acidic groups. Additionally by conversion to functionalities terminating in perfluoroalkyl groups the superhydrophobic properties of the closely spaced CF3 groups can be utilsed. In certain applications it is commercially attractive to change the surface properties of low cost materials such that they become superhydrophobic. For example cellulose of polyurethane foam are used for their absorbent nature in wound dressings and incontinence and other sanitary products. By virtue of the hydrophobic layer being present in the wicking effect can be directed and the flow of exudate or moisture constrained. Similarly for fluids with lower surface tension a superhydrophobic or oleophobic layer would offer the same mechanism.